1. Field of the Invention
This invention relates to an improved pressure swing adsorption process and apparatus for recovery of the more strongly adsorbed gas component in a multi-component gas mixture. More particularly, it relates to such a process and apparatus for the recovery of carbon dioxide from streams containing more weakly adsorbed components such as nitrogen, oxygen, hydrogen, methane, and carbon monoxide.
2. Description of the Prior Art
High-purity liquid carbon dioxide (99.99+%) is commonly produced by direct liquefaction of gas streams containing in excess of 95% CO.sub.2. These high-concentration sources are available directly as by-product streams from chemical processes such as ammonia synthesis. As a result, location of carbon dioxide liquefaction plants is traditionally dictated by the location and availability of these sources.
Many carbon dioxide customers do not require 99.99+% purity of liquefied CO.sub.2. Applications such as pH control and carbonate production can be effectively serviced with gaseous CO.sub.2 ranging in purity from 80% to 90%. Frequently, the sites where lower purity gaseous CO.sub.2 is needed have alternative sources of low concentration CO.sub.2 available, i.e., typically containing less than 20% CO.sub.2. Consequently, there is an opportunity for new technologies that are capable of economical on-site CO.sub.2 production from low-grade sources such as flue gas from boilers or other combustion sources.
Various methods for CO.sub.2 recovery from low and intermediate concentration sources are known. Chemical absorption of CO.sub.2 from a multi-component gas stream into a liquid absorbent, followed by heating to strip the CO.sub.2 from solution, is used to recover gaseous CO.sub.2 at 99+% purity. A variety of liquid amines, or potassium carbonate, can be used as the absorbent medium. The primary disadvantages of these processes are significant energy requirements for thermal regeneration of the absorbent, and reduction in absorbent capacity when modest quantities of oxygen are present in the multi-component gas stream.
Membrane separation processes may also be used for CO.sub.2 recovery, but these processes often require high feed pressures to achieve modest permeability for CO.sub.2. Expensive multi-staged membrane processes are needed for production of high purity CO.sub.2 from low concentration sources such as flue gas.
Pressure swing adsorption (PSA) separations offer significant potential advantages as compared with other methods for CO.sub.2 recovery. Thus, PSA offers the potential for lower-cost concentration and delivery of CO.sub.2 in comparison with the more traditional method of liquefaction and transportation of liquid CO.sub.2, particularly when transportation costs are high or attractive feed stocks for CO.sub.2 liquefaction are unavailable. A further and primary advantage of PSA techniques is the flexibility to produce CO.sub.2 product at variable purity. Adsorption and desorption pressures can be tuned, along with other process parameters, to yield the minimum desired product purity for a particular application. This allows power requirements to be reduced when high-purity product is not needed. A similar reduction in power consumption is difficult to achieve with liquid absorption processes since thermal stripping of the absorbent will always yield product at 99+% purity. PSA does not require a high temperature energy source like steam for regeneration, as do absorption processes. As a result, PSA is an attractive alternative for locations where steam is unavailable or expensive. In general, adsorptive separation is a reliable, flexible and potentially lower cost method for recovery of CO.sub.2, particularly when gas-phase purity in excess of 99% is not required.
For CO.sub.2 production from combustion flue gas, lime kiln off-gas, H.sub.2 plant tail gas and other sources, the function of the primary adsorbent(s) is to selectively adsorb CO.sub.2 while allowing lighter components to pass through. Water, which is typically more strongly adsorbed than CO.sub.2, may be present but can be effectively removed in a pretreatment layer of adsorbent. Therefore, the production of CO.sub.2 using PSA requires processes that are effective for heavy component recovery, that is, for recovery of the more strongly adsorbed component in a multi-component mixture.
A number of PSA processes for heavy component recovery, including the production of CO.sub.2 from low concentration sources, have been described in the prior art. See, for example, the following U.S. patents: Werner et al. U.S. Pat. No. 4,599,094; Fuderer U.S. Pat. No. 4,723,966; Lagree et al. U.S. Pat. No. 4,810,265; Hay U.S. Pat. No. 4,840,647; Schmidt et al. U.S. Pat. No. 4,892,565; Krishnamurthy et al. U.S. Pat. No. 4,963,339; Kumar U.S. Pat. No. 5,026,406; Knaebel U.S. Pat. No. 5,032,150; Kumar U.S. Pat. Nos. 5,248,322 and 5,354,346; LaSala et al. U.S. Pat. No. 5,370,728; Leavitt U.S. Pat. No. 5,415,683; and Couche U.S. Pat. No. 5,669,960. The most common applications for heavy component recovery are N.sub.2 /O.sub.2 separation utilizing zeolite adsorbents, and CO.sub.2 /N.sub.2, CO.sub.2 /CH.sub.4 and CO.sub.2 /H.sub.2 separations utilizing zeolites, activated carbons, silica gel or other adsorbents. Typically, prior art processes rely on compression of the feed to an elevated adsorption pressure, evacuation to recover heavy component product and rinsing with heavy component. Prior art processes typically use multiple beds to insure continuous utilization of equipment, with surge tanks used to dampen fluctuations in product flow and purity. Prior art processes for heavy component recovery, or combined light and heavy component recovery, can be divided into three general classes: conventional cycles, inverted cycles and reflux cycles. In a conventional cycle, adsorption occurs at higher pressure, with purging and recovery of heavy component product taking place at lower pressure. In an inverted cycle, adsorption occurs at lower pressure, with purging at a higher pressure. Each adsorbent bed in a reflux cycle contains a conventional bed portion and an inverted bed portion, with reflux of light and heavy component between beds. The advantage of the reflux cycle is that both light and heavy component products can be recovered at high purity and high recovery. However, this process is energy intensive and unattractive if recovery of the light component is not desired. The inverted cycle can be used to recover heavy component product at high purity, but requires significant power consumption. A conventional cycle may consume less power, but heavy component product purity varies throughout the cycle. An additional disadvantage of the inverted cycle is that it requires removal of water or other heavy components in a separate vessel before the feed enters the main adsorbent vessels. The individual steps in conventional PSA cycles are well known in the prior art. The first step in the basic cycle is adsorption, in which a multi-component feed gas is passed to the adsorbent bed at an elevated adsorption pressure. During this step the more selectively adsorbed component is retained by the adsorbent while the gas phase is enriched in less selectively adsorbed components. Typically, the adsorption step is terminated before the mass transfer front reaches the outlet of the adsorbent bed. Following adsorption, the adsorber vessel is depressurized via countercurrent blowdown and/or evacuation. As the pressure is reduced the gas phase becomes enriched in heavy component. At least a portion of the gas evolved during the depressurization stages is taken as heavy component product. Following depressurization, the adsorber vessel is repressurized to the adsorption pressure and the cycle is repeated. The basic cycle may be modified to include rinsing of the bed with heavy component product between adsorption and depressurization stages. This displaces a portion of the non-adsorbed gas from the bed and provides increased recovery of the heavy component product. The cycle may also include purging at intermediate or low pressures to further regenerate the adsorbent before the cycle is repeated.
Many of the potential on-site applications for gaseous CO.sub.2 are relatively small, e.g. less than 30 tons/day of contained CO.sub.2 product. On-site CO.sub.2 plants as small as 1 to 5 tons/day can be envisioned. This small plant size dictates the need for processes that are simple, reliable, and minimize process flow sheet complexity--and hence minimize capital cost. As plant size decreases, even relatively modest capital expenditures and fixed costs can add significantly to the unit production cost. The capital cost penalty for prior art processes with four or more beds, and associated valves, is significant when plant capacity is very small. The use of expensive surge tanks to dampen fluctuations in product purity adds additional cost to the process. Adsorption at elevated pressure, as in many prior art techniques, requires the compression of large amounts of the light (waste) components; this adds particular energy expense in recovering CO.sub.2 from dilute gas mixtures such as combustion flue gas which may contain as little as 6 to 10% CO.sub.2, i.e. energy is consumed in compressing 90% or more of the feed gas that is eventually discarded as waste.
Typical prior art processes for CO.sub.2 recovery have relied on adsorbents such as zeolite 13X or BPL activated carbon. For recovery of CO.sub.2 from low concentration sources such as flue gas, the advantage of using a relatively strong adsorbent such as zeolite 13X is that it retains a significant capacity for CO.sub.2, even at the low CO.sub.2 partial pressures present in the feed. The disadvantage is that it requires very low pressure for regeneration. BPL activated carbon is a much weaker adsorbent for CO.sub.2, and consequently, does not require such demanding desorption conditions. However, the utility of this adsorbent is diminished for low concentration sources like flue gas because of the weak and nonspecific interaction with CO.sub.2. At flue gas feed conditions, the equilibrium loadings of N.sub.2 and CO.sub.2 on BPL carbon are nearly identical, resulting in low adsorption selectivity. This low efficiency of separation severely limits the purity and recovery that can be achieved in the process.
It is among the objects of the present invention to provide improved PSA processes and apparatus for the recovery of heavy components such as CO.sub.2 from multi-component gas mixtures at predetermined substantially constant product purity, i.e. a variation of less than plus or minus 10 percent of the desired product purity, utilizing adsorption techniques employing low adsorption pressures and with no bed-to-bed interactions.
A further object of the invention is to provide such processes and apparatus, requiring lower capital and operating costs than for prior art techniques, particularly for small scale applications.
Yet an additional object of the invention is to provide improved PSA processes for the recovery of CO.sub.2 from multi-component gas mixtures, by utilizing as adsorbents therein zeolites having particular adiabatic separation factor and dynamic CO.sub.2 loading characteristics.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.